As structural materials having a high mechanical strength, irons or lumbers have been used for a long time. However, iron has a disadvantage of becoming rusty and a lumber has a disadvantage of being liable to corrode. Thus, synthesis resins have been used in place of the aforesaid materials.
In the case of making a light weight structural material using a synthetic resin, it has been conducted to form a foamed material by foaming a synthetic resin. As a foamed material for structural materials, a polystyrene resin, a polyethylene resin, or a vinyl chloride series resin has hitherto been used as the material therefor.
However, these materials are poor in heat resistance and also the strength thereof is relatively weak.
For solving the aforesaid problems, it is proposed to laminate a layer of a thermosetting resin layer such as a resorcinol series resin, etc., reinforced with fibers on one or both surfaces of a foamed material of a synthetic resin as a base material as described in JP-A-1-156051 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"). Also, in the proposed technique, a foamed material of polystyrene or a soft polyurethane is used as the foamed material of a synthetic resin. However, since the foamed material is originally soft or even if the foamed material is hard, the material is softened at about 100.degree. C., and also in the case of using a thermosetting resorcinol series resin, it is usually required to heat it to a temperature of at least 100.degree. C. while pressing for forming the layer of the resin, then by the aforesaid proposal, it is impossible to adhere the fiber-reinforced resin layer to the surface(s) of the foamed material while keeping the original form of the foamed material. Accordingly, a tough and lightweight synthetic resin structural material can not be obtained by the aforesaid proposal.
Also, a foamed material of PAT (thermoplastic polyester resin) is known. PAT is known as engineering plastics and has a large toughness, a good shape stability, and an excellent heat resistance capable of enduring 200.degree. C. Furthermore, since PAT is a thermoplastic resin, it may be considered that a foamed material of PAT can be easily obtained only by mixing a foaming agent with PAT and extruding the mixture through an extruding machine.
However, it is not easy to foam PAT different from polystyrene and polyethylene since PAT does not show a suitable viscosity for foaming at melting. The reason is that since PAT is a crystalline resin, when PAT is heated, PAT is suddenly softened to become a liquid having a low viscosity. Thus, PAT has a narrow temperature range suitable for foaming, whereby it is difficult to keep it at a temperature suitable for foaming, and since the viscosity of PAT is low, gases acting as foaming agent are immediately scattered and lost.
Thus, various attempts of easily producing foamed materials of PAT have been made.
For example, JP-B-56-8858 (the term "JP-B" as used herein means an "examined published Japanese Patent application") or U.S. Patent No. 4,462,947 proposes that PAT is mixed with a polycarbonate resin and the mixture is heated to a temperature of from 150.degree. C. to 250.degree. C. to generate carbon dioxide, whereby a foamed material of PAT is formed. However, since the foamed material thus obtained contains a polycarbonate resin, the foamed material is soft and pliable, and hence is not suitable as a structural material.
Also, JP-B-61-48409 proposes that the PAT is mixed with a diglycidyl ester and the mixture is foamed by extruding. However, the aforesaid patent publication shows only that a foamed material is obtained at a high expansion ratio of 15 times but any properties of the foamed material obtained are not clarified.
JP-B-61-484410 teaches that a PAT composition shall be foamed by extruding on considering the crystallizing speed of PAT. However, the method is limited to the case of obtaining strings having a sectional area of from 1 mm.sup.2 to 200 mm.sup.2 and also since the patent describes that the strings obtained can be easily drawn and heatworked, it is clear that the invention of the aforesaid patent does not aim at a structural material.
Also, JP-B-61-484411 teaches that a PAT composition shall be foamed by extruding under the condition that the crystallinity of PAT becomes at least 30%. However, the method is also limited to the case of obtaining strings having sectional area of 1 mm.sup.2 to 200 mm.sup.2 as in the aforesaid patent and it is described that it is required to increase the crystallinity and the strings obtained can be easily bent and they can be easily knitted and woven. Thus, the teachings of the aforesaid patent is useless for the production of structural materials.
As described above, it is known that PAT is a crystalline resin and PAT having a different crystallinity is obtained according to the manner of producing the resin. Also, it is known that as PAT having a larger crystallinity is used, a foamed material having larger roughness and heat resistance is obtained. Accordingly, it has only been attempted to obtain foamed materials having a large crystallinity.
Also, it is known that the crystallinity of PAT can be generally measured by the density, X-ray diffraction patterns, nuclear magnetic resonance spectra, etc., of the PAT resin. However, since a foamed material of PAT contains a large number of small bubbles therein, the crystallinity thereof can not be measured by these methods. Accordingly, the crystallinity of a foamed material of PAT was discussed but the method which could practically measure the crystallinity thereof was indistinct.